The purpose of this study is to determine: 1) the mechanisms of primary hypertriglyceridemia related to VLDL overproduction and/or defective VLDL catabolism related to lipoprotein lipase or hepatic lipase; 2) the interaction of the above with secondary forms of hypertriglyceridemia; and 3) the response of LDL size, composition and mass to changes that occur in VLDL.